In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or under-sea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometre of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance. It is also cost-effective when transmitting power under the sea, even for short distances, because the capacitance of undersea cables is much higher than that of overhead lines.
In such power transmission networks, converters are required at each interface between AC and DC systems to effect the required conversion, and one such form of converter is a voltage source converter. More particularly, when interconnecting one or more wind turbines, i.e. a wind farm, with a main AC grid the power transmission network may include a wind farm side voltage source converter which interfaces directly with the wind farm and a grid side voltage source converter that interfaces directly with the main AC grid. An overhead line or an under-sea cable, i.e. a DC transmission link, extends between the wind farm side and grid side voltage source converters.
Each of the wind farm side and grid side voltage source converters includes a plurality of series connected sub-modules that have at least one switching element which is connected in parallel with an energy storage device in the form of, e.g. a capacitor. The or each switching element may include a semiconductor device in the form of, e.g. an Insulated Gate Bipolar Transistor (IGBT), which is connected in anti-parallel with a diode although it is possible to use other semiconductor devices.
Switching of the or each switching element selectively directs current through the capacitor or causes current to bypass the capacitor such that each sub-module is selectively able to provide a voltage. In this manner it is possible to build up a combined voltage, via the insertion of the capacitors of multiple sub-modules (with each sub-module providing its own voltage), which is higher than the voltage available from each individual sub-module.
The sub-modules work together in this manner to provide a stepped variable voltage source. This permits the generation of an AC voltage waveform which enables the voltage source converter to provide the aforementioned power transfer functionality between a respective AC system, e.g. a wind farm or a main AC grid, and an associated DC system, e.g. a DC transmission link.
According to a first aspect of the invention there is provided a wind farm side voltage source converter comprising a DC terminal for connection to a DC transmission link; an AC terminal for connection to a wind farm including at least one wind turbine; and a main controller configured to modify an active power demand of the voltage source converter received from a higher level controller by introducing an artificial inertia factor and/or in response to a measured DC voltage at the DC terminal.
Modifying the active power demand of the voltage source converter by introducing an artificial inertia factor compensates for a lack of mechanical resistance in the or each wind turbine of the wind farm. This, in turn, helps to resolve instability issues in a main AC grid where the power output of the wind farm is evacuated which might otherwise be caused by operation of the voltage source converter. It also helps to smooth out variations in the frequency at which the voltage source operator operates.
Meanwhile, modifying the active power demand of the voltage source converter in response to a measured DC voltage at the DC terminal allows a DC transmission link which is, e.g. operatively associated in use with the wind farm side voltage source converter, to act as a communication medium. This in turn allows, e.g. a grid side voltage source converter connected to the other end of the DC transmission link, to control the active power output of the wind farm side voltage source converter without the need for a separate costly and unreliable telecommunication system between the wind farm side voltage source converter and, e.g. the grid side voltage source converter.
In an embodiment, the main controller includes a power-frequency slope sub-controller and the main controller is configured to modify the active power demand of the voltage source converter by passing the active power demand through the power-frequency slope sub-controller and thereafter applying an artificial inertia factor to the resulting frequency.
Such a step helps directly to prevent the resulting frequency, i.e. the frequency value output by the power-frequency slope sub-controller and the frequency at which the wind farm side voltage source converter is controlled to operate, from varying too erratically.
The power-frequency slope sub-controller may be configured itself to apply the artificial inertia factor.
Such a configuration can be readily implemented by minor alteration of the operating algorithms of the power-frequency slope sub-controller.
Optionally the artificial inertia factor takes the form of a lagging function.
A lagging function desirably effects the rate at which the frequency value output by the power-frequency slope sub-controller varies.
In an embodiment of the invention the main controller is configured to modify the active power demand of the voltage source converter received from a higher level controller in response to a measured DC voltage at the DC terminal by comparing the measured DC voltage with an expected DC voltage based on a DC voltage demand received from a higher level controller and altering the active power demand when the measured DC voltage differs from the expected DC voltage.
Having a main controller configured in the foregoing manner allows the main controller to vary the active power output by the wind farm side voltage source converter so that it is aligned with the active power demand of, e.g. a grid side voltage source converter which in use is connected with the wind farm side voltage source converter by a DC transmission link, such that, e.g. the grid side voltage source converter is able to act as a master converter while the wind farm side converter acts as a slave converter.
The altered active power demand may be passed through the power-frequency slope sub-controller before an artificial inertia factor is applied to the resulting frequency.
Such an arrangement allows for the implementation of a master-slave control relationship between the wind farm side voltage source converter and, e.g. a grid side voltage source converter, in a manner whereby the wind farm side voltage source converter remains stable and subject only to smooth variations in operating frequency.
According to a second aspect of the invention there is provided a first electrical system comprising a wind farm side voltage source converter as described hereinabove having a first end of a DC transmission link connected to the DC terminal thereof; and a grid side voltage source converter including a DC terminal to which a second end of the DC transmission link is connected and an AC terminal for connection to a main AC grid. The main controller of the wind farm side voltage source converter is configured to compare the measured DC voltage at the DC terminal of the wind farm side voltage source converter with an expected DC voltage defined by the sum of a DC voltage demand of the gird side voltage source converter received from a higher level controller and the voltage drop along the DC transmission link; and alter the active power demand of the wind farm side voltage source converter received from the higher level controller when the measured DC voltage differs from the expected DC voltage.
Such an arrangement allows the DC voltage demand of the grid side voltage source converter, i.e. as determined by a higher level controller such as a dispatch centre, to control the active power output of the wind farm side voltage source converter (e.g. so that it matches the active power demanded by the grid side voltage source converter, without the need for a separate telecommunication system).
According to a third aspect of the invention there is provided a power transmission network comprising a plurality of parallel-connected wind farm side voltage source converters as described hereinabove, and/or a plurality of parallel-connected first electrical systems as described hereinabove.
Such a power transmission network allows a large wind farm, i.e. a wind farm which includes a high number of wind turbines such that the maximum potential power output of all of the wind turbines is much greater than the power a single wind farm side voltage source converter can handle, to feed a respective plurality of wind farm side voltage source converters and have them share the power output between themselves according to their individual capabilities, while at the same time maintaining the overall stability of the power transmission network.
Moreover, since each wind farm side voltage source converter has a main controller which includes a power-frequency slope sub-controller and which is configured to apply an artificial inertia to the frequency resulting from the said sub-controller, the or each remaining wind farm side voltage source converter in the power transmission network can easily, i.e. without the need for a complex additional control system, continue to be frequency controlled by its associated main controller in the event that, e.g. one or more of the wind farm side voltage source converters experiences and outage and goes offline.
In another wind farm side voltage source converter according to a further embodiment of the invention the main controller is configured to modify an active power demand of the voltage source converter in response to a measured DC voltage at the DC terminal by comparing the measured DC voltage with a predetermined DC over-voltage value and reducing the active power demand when the measured DC voltage is greater than or equal to the DC over-voltage value.
Such a main controller assists the wind farm side voltage source converter to reduce the active power it outputs, e.g. and thereafter in use transmits to a grid side voltage source converter via an associated DC transmission link, in response to a change in the DC voltage presented at the DC terminal of the wind farm side voltage source converter. Such a change in the DC voltage may be occasioned by a fault in another part of the power transmission network in which the wind farm side voltage source converter is in use located, and so the foregoing arrangement permits the wind farm side voltage source converter to react to the fault without the need for a separate telecommunications system between the wind farm side voltage source converter and the other part of the power transmission network.
According to a fourth aspect of the invention there is provided a second electrical system comprising a wind farm side voltage source converter as described hereinabove having a first end of a DC transmission link connected to the DC terminal thereof; and a grid side voltage source converter including a DC terminal to which a second end of the DC transmission link is connected, an AC terminal for connection to a main AC grid, and a second main controller configured to detect an increase in the frequency at which the grid side voltage source converter is operating and thereafter increase the DC voltage at the DC terminal of the grid side voltage source converter to the predetermined over-voltage value.
The frequency at which the grid side voltage source converter operates will, in use, have a tendency to increase in the event of a fault in the main AC grid which gives rise to an islanding of the second electrical system, i.e. a separation of the second electrical system from the main AC grid, e.g. by the tripping of a remote breaker elsewhere in the main AC grid. As a result, the inclusion of a second main controller which is able to increase the DC voltage at the DC terminal of the grid side voltage source converter to the predetermined over-voltage value in response to such an increase in the operating frequency of the grid side voltage source converter, provides the option of selectively reducing the active power flowing through the DC transmission medium from the wind farm side voltage source converter to the grid side voltage source converter, and at the same time provides the option of controlling the increase in the frequency at which the grid side voltage source converter is operating to within a safe limit.
In addition, such functionality and the protection of both the grid side voltage source converter and the wind farm side voltage source converter are provided without the need for a separate telecommunication system between, e.g. the remote breaker and one or both of the wind farm side voltage source converter and the grid side voltage source converter.